The invention relates to a wearing part. More particularly the invention relates to a cutting insert made of hard metal and comprising a multi-layered coating of hard material having at least one layer that is an oxide layer, and which is employed in metal cutting work.
A wearing part of the type generally described above is disclosed in German Auslegeschrift 22 53 745, in which the inner layer adjoining the basic, hard, metal body is composed of one or a plurality of carbides and/or nitrides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Si and/or B, and the outer layer is composed of one or a plurality of highly wear-resistant deposits of alumina and/or zirconia.
The disclosed wearing part is disadvantageous to the extent that cracks may form in the top layers of pure oxide and the oxide layers exhibit in many cases insufficient adhesive strength and, consequently, peel off. The friability of the oxide layer increases strongly as the thickness of the layer is increased and causes a highly disadvantageous change in the structure, so that as a practical matter, such layers on such wearing parts are limited to a comparatively very low thickness of only a few micrometers, i.e., a thicker layer does not bring any additional advantages. This, in turn, decisively limits the wear life of such wearing parts, such as, for example, of reversible cutting attachments for metal cutting.
German Offenlegungsschrift 23 17 447, which represents an application of addition or improvement to the above-mentioned German Auslegescrift 22 53 745, specifies a wearing part having an outer top layer which is composed of one or a plurality of deposits of ceramic oxides, and, in addition to the oxides disclosed in the main patent lists, oxides of the elements Si, B, Ca, Mg, Ti and/or Hf, generally including in the application also the formation of mixed oxides. No special mention is made of any individual mixed oxides.
To the extent to which experience has been gained with individual embodiments in practical application, the occurrence of cracks and the adhesive strength of the top oxide layers is not satisfactory in any of these cases.
A composite body preferably comprising a basic body made of hard metal is known from German Auslegeschrift 28 51 584, in which one or a plurality of layers composed of one or a plurality of carbides and/or nitrides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Si and B are arranged on the basic body, and on which one or a plurality of layers, there is arranged one or a plurality of layers composed of a mixture of at least one oxide and at least one nitride and/or at least one oxynitride of the elements Cr, Al, Ca, Mg, Th, Sc, Y, La, Ti, Hf, V, Nb, Ta; with the nitrogen content of the outermost layer being in a range of from about 0.1 to about 30 atom-%, and preferably in a range of from about 0.2 to about 15 atom-%. The single example specifies the following structure of the layer on hard metal: TiC, 4 .mu.m+Al.sub.2 O.sub.2.8 N.sub.0.2, 2-3 .mu.m.
Primarily hard-metal wearing parts are known and in practical commerical use in which the outer layer is composed of a relatively large number of alternating layers of Ti(C,N) and Al.sub.2 (O,N).sub.3. Such wearing parts are within the scope of German Auslegeschrift 29 17 348. However, with such composite bodies, the resistance to wear which can be achieved is not satisfactory for many cases of application. Furthermore, the excessive number of individual layers--the single example of German Auslegeschrift 29 17 348 specifies 38 individual layers--is not economical in terms of manufacture.
Therefore, it is the object of the present invention to provide a wearing part, in particular a cutting insert made of hard metal for metal cutting, which has a multi-layered coating of hard material, in which at least one layer is an oxide layer and which has an improved resistance to wear and which exhibits enhanced adhesive strength with respect to the hard material coating as compared to known wearing parts.